Catalytic vapor phase oxidation of n-butane to maleic anhydride in heat transfer, medium-cooled, tubular reaction zones is well-known. Typically, a gaseous feed comprising molecular oxygen, n-butane and ballast gas is passed over a fixed bed of oxidation catalyst in one or more reaction tubes at temperatures of about 300.degree. C. (572.degree. F.) to about 650.degree. C. (1202.degree. F.) and pressures of from about 10 to 75 psia. In general, catalysts for the oxidation of C.sub.4 hydrocarbons to maleic anhydride are based on vanadium and phosphorus. The difficulty with phosphorus-vanadium metal-promoted catalysts is that they tend to deactivate very quickly. It has been found by prior investigators that, contrary to some common reactions wherein the catalyst loses activity with time and the temperature is raised to compensate and to maintain the desired activity, in the oxidation of butane to maleic anhydride with vanadium-phosphorus catalysts, catalyst activity increases and selectivity decreases, with consequent loss in yield of maleic anhydride and increased production of carbon oxides and water. Since the oxidation reaction is highly exothermic, increased catalyst activity tends to increase oxidation temperature with consequent further increase in catalyst activity and decreased selectivity. In order to maintain the desired reaction zone temperature, heat transfer medium such as an oil or molten salt is circulated around the reaction tube or tubes. Typically, temperature of the heat transfer medium is adjusted to provide adequate cooling at the hottest point of the reaction zone. Given the positive dependency of reaction rates on reactant concentrations, the hottest point of the reaction zone is located where the reactant concentrations are greatest. Ideally, the hottest point of the reaction zone is maintained at a temperature wherein concentration of reactants is at a level to provide maximum conversion of the reactants and maximum selectivity to maleic anhydride without thermal runaway.
At present, known commercial processes for producing maleic anhydride from n-butane can be characterized as once-through air-oxidation processes in that air is used as the source of molecular oxygen, and, owing to the nitrogen content of air, levels of nitrogen in the reaction zone effluent build up to such an extent that recycle of effluent, with or without separation of nitrogen from unreacted n-butane, is economically impractical. In view of the flammability of mixtures of n-butane and air, concentrations of n-butane in a once-through air-oxidation process are limited typically to about a maximum of 1.8 mole %. Due to the impracticality of recycling unreacted n-butane at very low concentrations, the unreacted n-butane typically is discarded, further reducing total yield from n-butane consumed.
Suggestions have been proposed in the prior art to prepare maleic anhydride from n-butane in a continuous method wherein n-butane concentrations are higher than in the above once-through air-oxidation process. Substantially pure (at least 95%) molecular oxygen is used, and the temperature of the hottest point of the reaction zone, the so-called "hot spot", is controlled by the heat transfer medium to provide cooling. Continuous phosphorus addition is taught to maintain yield.
U.S. Pat. No. 4,342,699 teaches a batch process for production of maleic anhydride comprising contacting n-butane-rich feed consisting essentially of n-butane, molecular oxygen and ballast gas with an oxidation catalyst comprising a co-metal promoted vanadium-phosphorus catalyst. The co-metal is zinc, bismuth, copper or lithium. The reaction was in a heat transfer, medium-cooled tubular reaction zone, containing a fixed bed of catalyst graded in terms of reactivity wherein (1) n-butane is oxidized at relatively low per pass conversions, (2) reactor effluent is withdrawn from the reaction zone and a major portion of maleic anhydride is separated therefrom, and (3) a major portion of the effluent remaining after separation is recycled to the reaction zone. The feed consists essentially of about 2 to 10 mole % n-butane, about 8 to 20 mole % molecular oxygen and a balance of inert gas or gases. Air is taught as used on process start-up until inerts in the recycle gas build up, although oxygen instead of air can be used. An effluent comprising maleic anhydride, oxygenated by-product, unreacted n-butane and oxygen, and inert gas or gases is withdrawn from the exit end of the reaction zone, maleic anhydride and oxygenated hydrocarbon by-products are separated therefrom, and the remaining effluent is separated into a purge stream that is removed at a rate that substantially compensates for the buildup of inerts in the reaction zone. A recycle stream is recycled to the reaction zone with addition of make-up gas comprising n-butane and molecular oxygen. Reported yields ranged from 61.2 wt. % to 91.4 wt. % with recycle using pure oxygen. Runs of limited duration were from 1 to 4 days. Yields of up to 91.4 wt. % were achieved only by periodic regeneration of the catalyst by passing carbon tetrachloride over the catalyst. Accordingly, yields of about 90 wt. % are achieved with intermittent process down periods for catalyst regeneration. U.S. Pat. No. 4,342,699 does not teach or suggest a continuous process of extended duration with consistent high yields of at least 90 wt. %. Despite the use of recycle, U.S. Pat. No. 4,342,699 does not teach or suggest a continuous process for oxidation of n-butane to maleic anhydride.
U.S. Pat. No. 4,649,205 teaches a continuous process using air as a source of oxygen for oxidation of n-butane to maleic anhydride wherein the catalyst is continuously regenerated by contacting it during the vapor-phase oxidation with a phosphorus compound and water. The feedstock contained from 0.2 to about 1.7 mole % butane. The reaction was on a once-through basis without recycle. Yields were increased by addition of phosphorous compounds from about 80 wt. % to about 92 wt. %.
U.S. Pat. No. 4,795,818 teaches the continuous addition of a phosphorus compound to maintain reaction temperature at a constant level. Recycle operation is simulated in Examples 6 and 7. Feed range was 2.4-5.5 vol. % n-butane, 10-12 vol. % oxygen and the balance nitrogen. The process comprised operating the process in the absence of added phosphorus compound at a preselected conversion to obtain a specific yield of maleic anhydride, determining the temperature of the reaction at that conversion and yield and thereafter maintaining the temperature substantially constant by controlled continuous addition of a volatile phosphorus compound to maintain the yield of maleic anhydride substantially constant. Operation was taught as on a single-pass basis for 600 hours. Catalyst operating temperature was taken at the outlet gas temperature. Conversion of butane was 25% with 65% selectivity to maleic anhydride. Nitrogen buildup in recycle was not disclosed. No provision was taught for recovery of n-butane in the effluent or recycle stream.
Despite the improvements reported in the above-described patents, these improvements are not entirely satisfactory from the standpoint of operation of a continuous process for long periods to prepare maleic anhydride from n-butane wherein recycle is required to recover unreacted n-butane. With recycle, impurities in the n-butane feed and carbon oxides from the oxidation reaction build up in the recycle stream to reduce oxygen partial pressure and reduce yield of desired product.
Although the prior art suggest that recycle of effluent from a maleic anhydride oxidation process, coupled with addition of a phosphorus compound in water, can result in a viable, long-term, continuous oxidation process, such has not been demonstrated heretofore wherein product yield and catalyst productivity are equal to or better than previously obtained product yields and catalyst productivity.
It has been found that continuous operation for extended periods to achieve high product yields of maleic anhydride and desirable catalyst productivity requires highly purified oxygen as the oxidizing agent, limitation of oxygen and n-butane in the feed stream to assure safety, limited conversion of oxygen and n-butane in the oxidation process to reduce production of carbon oxides as by- products, addition of a phosphorus compound and water to maintain catalyst selectivity, and use of a co-metal modified catalyst of vanadium-phosphorus-oxygen wherein the co-metal is preferably molybdenum.
The use of air as a source of oxygen in a continuous recycle oxidation process is not feasible because of nitrogen build-up in the effluent.
The presence of carbon monoxide in quantity in the recycle stream as an oxidation product, and its subsequent undesirable oxidation to carbon dioxide, liberates heat and increases reaction temperature with consequent increased hot-spot temperature and consequent possible runaway hot-spot exotherm. Accordingly, catalyst temperature control, within narrow limits, is required in a continuous process for oxidizing n-butane to maleic anhydride.
Highly pure oxygen as an oxidizing agent, instead of air, is required to limit the presence of nitrogen in the process effluent stream. Highly pure oxygen, however, can be present in the oxidation reaction only in limited amounts to limit the reaction of carbon monoxide to carbon dioxide with production of excess reaction heat, thus increasing reaction temperature and hot spot temperature. Conversion rates of oxygen and n-butane are hot-spot temperature dependent, i.e. a high hot-spot temperature increases conversion of oxygen and n-butane.
Use of co-metal modified catalyst of vanadium-phosphorus-oxygen wherein the co-metal is molybdenum is required for increased yield and catalyst productivity over that obtained with a commercially available catalyst. Catalyst selectivity requires addition of a phosphorus compound and water.